In general, the manhole is installed for maintenance of facilities for water service/sewerage, electrical communication etc., which are laid under the ground, or draining of rainwater, and mainly consists of circular or rectangular structure.
However, since the existing manhole cover cannot be adjusted in its height, the height between slab of manhole structure and road surface is not constant for each manhole, and thus, in the installation of most manhole covers, in order to make the manhole cover flush with the road surface, a space between lower portion of the manhole cover and the slab of the manhole structure is filled with height-adjusting material such as bricks. Therefore, there is a problem that the height-adjusting material such as bricks is destroyed and sinks downward due to reduction of strength of the bricks etc. resulting from ingress of rainwater etc. and fatigue load such as repeated impact load of the vehicle after completion of construction, and accordingly frequent repairing of the manhole cover on the road is required, which makes management of the manhole difficult.
In order to solve the above-mentioned problems, in the prior art, as shown in Patent Registration No. 10-0522941 of the applicant, spiral rib formed on outer surface of an upper body, on which a lid of the manhole cover is mounted, rides over and supported by spiral rib formed on inner surface of a lower body and is secured with bolts, whereby the upper body is supported by the lower body. Irregularities are formed on each spiral rib; the irregularities formed on the lower side of the rib of the upper body and the irregularities formed on the upper side of the rib of the lower body engage with each other, thereby securing the upper body to the lower body. Furthermore, pitch of the rib is made sufficiently large, hence the upper body can be easily rotated. The upper body can be secured to and supported by the lower body with no slip. However, in the case of forming a single spiral rib only, horizontal angle of the rib may not be made on a plane and a certain degree of taper is generated due to problems occurring in the processes of molding and releasing of castings, and thus there is a problem that a minute play may occur due to the taper in coupling of the upper and lower bodies.
Furthermore, when the bolts are to be fastened to the upper body in order to prevent the play in coupling of the upper and lower bodies, necessary bolt-fastening holes should be formed, however, since the height of the upper body should be adjusted to make it flush with the road surface, the bolt-fastening holes are difficult to form or should be formed in a great number in the process of manufacturing in factory. Therefore, there are problems associated with the manufacturing or operations on site are cumbersome, and there is a problem that since the bolt-fastening holes are scattered rainwater may ingress into the bolt-fastening holes in the case of the manhole requiring the waterproof.
Furthermore, since the upper body is secured to the lower body by means of the spiral rib only, the play occurs between the upper and lower bodies, and since the bolts securing the upper body to the lower body are installed in such a way to be connected to the lower body through the inside of the upper body, user should install the bolts while inclining into the inside of the upper body in order to install the bolts, hence there is a problem that installation of the bolts is very inconvenient.
Furthermore, since only single spiral rib is formed, displacement by rotation is longer in adjustment of the height, hence there is a problem that waste of manpower, equipment and time occurs.
Furthermore, since waterproof of the manhole cover cannot be smoothly achieved, there is a problem that rainwater may ingress into the inside of the manhole through the manhole cover.